The present invention relates to improvements in pumps having a piston displaceably supported in a pump cylinder for aspirating a pressure fluid via a feed line and for expelling the pressure fluid into a line, particularly a brake line.
Pumps of this kind are used especially in ABS brake systems. As a rule, the pumps serve to pump brake fluid back into a fluid tank or reservoir.
It is self-evident that in operation of a brake system, dirt, abraded particles, oil deposits or the like collect in the brake fluid and can impair the operation of the brake system. For this reason, the brake fluid should be passed through filters during its circulation.
One such filter is located in the aforementioned pump, for instance, and shields the feed line from the suction chamber. Such a filter is suggested, for instance, in U.S. Pat. No. 4,887,870. Such a filter can also be found in German Published Patent Application No, 32 36 536. In the corresponding annular chamber, these filters form an annular gap toward the pump housing, so that fluid is present in this annular gap, upstream of the filter. However, since in the known pumps the pump cylinder rests with a stop ring, upstream of the annular chamber, on a housing shoulder with a precisely defined bearing surface area which cannot be altered, enlarging this annular gap presents considerable difficulties, yet the enlargement is desirable to improve the suction. In a larger annular gap, there is more fluid present upstream of the filter and subjected to the pumping action.
Altering the annular gap can be done only by shifting the bearing face outward, but this would necessitate enlarging the diameter of the pump cylinder and hence of the entire pump.